The present invention relates to actuators for valves, and, more particularly, to coupling systems used to connect rotary position actuators with rotary controlled valves.
Numerous types of valves exist to turn fluid flow on and off and regulate fluid flowrates within a piping system or flow conduit. Some valves restrict flow with an axial movement or displacement of a valve element within the housing or valve body. For instance, most spool valves and globe valves restrict flow with an axial displacement of a needle, plug or spool within the valve body. The force causing the displacement of the valve element may be provided in a number of ways, such as with hydraulic, pneumatic or other pressure control on a different portion of the valve element. Other valves operate based on a rotary movement or pivoting of a valve element relative to the housing. For instance, many ball valves and rotary valves operate based on a rotating a ball or spool relative to the housing without any displacement between the two. Even with displacement type valves, the axial displacement may occur as a result of a screw threaded advance, and thus adjustment of the valve occurs primarily due to a rotary control motion. The present invention is applicable to all types of rotary controlled valves, and particularly applicable to rotary control valves without axial displacement, such as ball valves.
Within the past several decades it is become common to employ automated control systems to operate valves rather than requiring manual control. The automated control systems include an actuator for each valve in the system. The actuator may include an electrically powered motor or solenoid which controls the position of the valve element based on electrical input to the actuator.
With some valves such as most globe valves, the flow through the valve is directly, linearly proportional to the position of the valve element. With other types of valves, such as most ball valves, flow has a non-proportional relationship to the angular position of the ball. Early on in the use of automated control systems, proportional valves were used for all instances when flowrates had to be controlled, and non-proportional valves were used for instances when flow between open and shut positions was sufficient. More recently, pressure and flow sensing devices have become more sophisticated and have enabled fine tuning of non-proportional valves using downstream feedback. With use of downstream pressure or flow feedback, non-proportional valves may now also used in providing controlled flowrates.
Often it is desired to retrofit existing piping systems to include an automated control system. The retrofit involves adding actuators to open, close and/or adjust valves in place of previous manually moved handles.
Particularly when retrofitting a valve in an existing piping system with an actuator, it is difficult to precisely align the actuator with the stem of the valve. Misalignment may occur with a displacement component, when the shaft coupling the actuator to the valve is displaced from the axis of the valve stem, even through the shaft axis is parallel to the valve axis. For instance, some valves already include a flange with threaded holes to facilitate mounting of a handle or other external device, and the flange may be used for mounting of the actuator. However, the valve stem axis may not be exactly centered between the threaded holes on the flange. Misalignment may also occur with an angular offset, when the shaft coupling the actuator to the valve stem is disposed at an angle to the valve stem. For instance, the plane formed by the flange may not be exactly perpendicular to the valve stem axis. Some valve stems include flats to facilitate rotating the valve stem, but these flats may not be entirely parallel to each other and equally spaced on opposite sides of the valve stem axis. The valve stem itself may not be aligned with the axis of rotation of the valve element, and may not perfectly rotate about its axis. Any of these problems can result in misalignment between the actuator and the valve stem. Some misalignments include both a displacement component and an angular offset component.
When the valve is manually turned, these existing inaccuracies may not pose major problems. Manual handles are typically mounted directly to valve stems, limiting the effect of any angular offset. The manual handle is left free for grasping, and rarely transmits a residual stress. Forces transmission to the valve stem is generally not exactly reproduced from rotation to rotation, so any wear problems associated with misalignment are not focused at a particular location. Manual turning also has a great capacity to adjust the turning torque appropriately for the turning force required.
In contrast, when an automated control system with an actuator is used to turn the valve, any misalignment between the shaft and the valve stem becomes more significant for a number of reasons. First, the shaft extends the valve stem a significant distance, and any angular offset at the valve stem results in a large difference at the actuator. A longer distance between the actuator and the valve stem and a longer shaft for turning the valve stem exacerbates the displacement problems and particularly angular misalignment problems. Second, in contrast to the manual handle, the shaft does not terminate in a free end, and misalignment will often result in a residual stress or bending moment on the valve stem. That is, if the shaft is misaligned to place a bending moment on the valve stem, that bending moment will be constant, and will not relax just because the valve is not being moved. Third, the actuator places forces on the valve stem which are exactly reproduced for each turning of the valve stem, resulting in more focused wear problems. Fourth, if the valve stem itself does not rotate perfectly about its axis, the shaft may place a bending moment on the valve stem with a magnitude that changes upon the rotational location of the valve. For instance, the valve stem may be fairly free of residual stress when the valve is closed, but have a severe bending moment when the valve is open. Fifth, because the automated control system is intended to more tightly control of the flow through of the piping system, the valve is likely to be operated much more frequently.
The resulting misalignment can cause a variety of problems in the piping system and/or the automated control system. As the valve stem is repeatedly and continually stressed over time, the valve stem may warp or be broken off entirely, rendering the valve unworkable. If the automated control system does not have feedback sensors in place, such breakage may not be readily identified, and the actuator may continue to turn the shaft even though the valve element is not being moved. More likely than breakage, the seals around the valve stem are likely to wear excessively and start leaking. If the shaft "binds" or torques differently depending on the position of the valve, the actuator may have trouble turning the shaft, or may not turn the shaft an appropriate amount corresponding to the input signal.